1. Field of the Invention
The present invention relates to a door impact beam contained in a door of an automobile, and more particularly, to a door impact beam enjoying a good impact absorbing capability.
2. Description of the Related Art
Door impact beams are known as means for reinforcing doors of automobiles. The door impact beams are also referred to as door guard bars. A conventional door impact beam is formed by press-molding a high-strength steel plate into a predetermined shape. This plate-shaped door impact beam is arranged in a door panel structure of an automobile, along the longitudinal direction thereof, and serves to augment the rigidity of the door against a load applied sideways thereto. In case of a flank clash, energy which acts on the door is absorbed by plastic deformation of the door impact beam, whereby users' safety is secured.
Since it is bound to be arranged in the narrow inside space of the door, the conventional plate-shaped door impact beam must be formed of a plate which is wide enough to ensure predetermined energy absorbing capability and door deformation. In this case, the unit plate is considerably heavy, weighing about 5 to 8 kg.
Accordingly, use of a heat-treated steel pipe as a material of the door impact beam has been proposed to reduce the mounting space and weight of the impact beam. The conventional pipe may be a round pipe having a circular section or a square pipe a having a square section, as shown in FIG. 12.
The strength of the door impact beam is evaluated by using a testing apparatus, such as the one shown in FIG. 13. In this testing apparatus, a load W equivalent to energy produced by a lateral clash is applied to the central portion (with respect to the lengthwise direction) of a beam d, as an object of testing, by means of a pressure member c. As the load W increases, in this case, the beam d bends in the manner shown in FIG. 14, and undergoes a U-shaped plastic deformation. Thus, the beam d touches the pressure member c on points A and B for a tangential angle .theta..
The geometrical moment of inertia of a round pipe is smaller than that of a square pipe of the same outer dimensions and the same wall thickness. In the case of the round pipe, therefore, the initial leading edge gradient of a characteristic curve P2, which represents the relationship between deflection and load, is relatively gentle, as indicated by dashed line in FIG. 15, so that an unreasonable deflection is allowed in a moment of a clash. Thus, the round pipe requires a larger sectional area than that of the square one, thereby entailing an increased weight and the like, in order to obtain the same strength as the square pipe which has the same onter demensions.
The square pipe a shown in FIG. 12 sometimes suffers a sudden buckling when it is subjected to a measure of deflection after a load equivalent to the energy of a lateral clash is applied to its central portion with respect to the lengthwise direction thereof. Thus, the impact absorbing capability of this pipe is lowered by a smaller deflection than in the case of the round pipe. When a conventional square pipe (H=32 mm, wall thickness t=3 mm, span L=700 mm) with a square section, for example, was bent by means of the aforesaid testing apparatus, it displayed a characteristic curve P1 indicated by full line in FIG. 15, which indicates a breakage by a deflection of 100 mm or thereabout. This level of deflection cannot ensure satisfactory reinforcement of the door. With use of the round pipe having the same sectional area and the same outer dimensions as the square one, however, the leading edge gradient of the characteristic curve P2 is so gentle that the undue deflection is allowed in the moment of the clash, although no buckling is caused. In consequence, round pipes with wide sectional areas are liable to be used, thus resulting in an increase in weight.